The present invention relates to a method of pulling up a nitrogen-doped silicon single crystal, which is intended for use in the manufacture of a semiconductor device, and to a method of manufacturing an epitaxial wafer using a silicon wafer prepared from a silicon single crystal produced by that method. More particularly, it relates to a method of manufacturing a high-quality epitaxial wafer while scarcely giving rise to stacking faults, dislocations and other defects in an epitaxial layer (hereinafter collectively referred to as xe2x80x9cepitaxial layer defectsxe2x80x9d) when it is grown on a wafer sliced from a nitrogen-doped silicon single crystal, and to a method of producing such a single crystal to serve as a raw material for the epitaxial wafer.
In the art, silicon single crystals for use in manufacturing semiconductor devices are produced by the Czochralski method (CZ method). FIG. 1 is a sectional view schematically illustrating a producing apparatus used in the CZ method. The producing apparatus comprises a crucible 1 disposed in the middle of the apparatus, and the crucible comprises a quartz vessel 1a and a graphite vessel 1b with the quartz vessel closely fitted therein. A heater 2 is disposed in a manner surrounding the crucible 1 and a raw silicon material is contained in a molten form 3 as melted by the heater. Above the crucible 1, there is suspended a pulling shaft 4 with a seed crystal 5 mounted thereon, and the shaft pulls up a single crystal 6 while allowing it to grow from the lower end of the seed crystal 5. A heat shield 7 is disposed in a manner surrounding the growing single crystal 6.
With the recent increase in the integration density of silicon semiconductor devices, quality requirements imposed on silicon wafers on which devices are formed have become more and more severe. For example, severer limitations are imposed than ever on dislocations and like crystal defects and/or metal impurities in the so-called xe2x80x9cdevice active regionxe2x80x9d where devices are formed, with the increasing fineness of circuits as resulting from the increase in integration density, since such defects and impurities increase the leakage current and shorten the life of a carrier.
Wafers sliced from silicon single crystals produced by the CZ method generally contain about 1018 atoms/cm3 of supersaturation oxygen. Due to the thermal history in the steps of device formation, such oxygen forms oxide precipitate nuclei and thereby induces crystal defects such as dislocations and stacking faults. In the process of device manufacture, however, the so-called DZ layer (denuded zone) which is free of crystal defects and has a thickness of about tens of micrometers is formed near the wafer surface by diffusion of oxygen to the outside when the wafer is maintained at about 1100xc2x0 C. for several hours in the step of field oxide film formation by LOCOS (local oxidation of silicon) or well diffusion layer formation. The DZ layer serves as a device active region, so that the occurrence of crystal defects is spontaneously prevented.
However, with the increase in the integration density of semiconductor devices, the high-energy ion implantation technique has been introduced for well formation by which the device process is carried out at a low temperature of 1000xc2x0 C. or less. At such a temperature, the above-mentioned outward diffusion of oxygen does not occur to a sufficient extent, hence the DZ layer formation near the surface becomes insufficient. Therefore, attempts have been made to reduce the oxygen content in wafers, but such attempts have been unsuccessful in perfectly suppressing the formation of crystal defects.
Under such circumstances, epitaxial wafers having an epitaxial layer substantially free of crystal defects as formed therein have been developed and are now widely used in the manufacture of highly integrated devices. However, even epitaxial wafers high in crystallinity are used, the device characteristics are degraded due to contamination of the epitaxial layer with metal impurities in the subsequent device process steps.
The opportunity and influences of such contamination with impurity metal elements increase since the process becomes more complicated with the increase in integration density. The contamination may be eliminated basically by cleaning the process environment and materials used. However, it is difficult to render the device process completely free of contaminants, hence the gettering technology becomes necessary as a measure for solving that problem. This is a means for entrapping contaminant impurity elements in a region (sink) other than the device active region to thereby render the contaminants harmless.
The gettering technology includes intrinsic gettering (hereinafter referred to as xe2x80x9cIGxe2x80x9d for short) which comprises entrapping impurity elements by utilizing oxygen-caused oxide precipitates spontaneously induced during heat treatment in the device process steps. However, when a wafer is heat-treated at a temperature as high as 1050-1200xc2x0 C. in the epitaxial step, oxide precipitate nuclei occurring within the wafer sliced from a silicon single crystal shrink and vanish, whereby it becomes difficult to sufficiently induce oxide precipitates to serve as gettering sources within the wafer in the subsequent device process steps. Thus, even if the gettering technology is applied, a problem arises that any satisfactory IG effect cannot be exerted on metal impurities throughout the whole process.
To overcome such a problem, methods of producing silicon single crystals have been proposed in the art which comprise doping the single crystals with nitrogen while they are grown by the CZ method, to thereby induce formation, within wafers, of oxide precipitate nuclei hardly vanishing even upon high temperature heat treatment in the epitaxial step (cf. e.g. Japanese Patent Application Laid-Open (Kokai) No. H11-189493 and Japanese Patent Application Laid-Open (Kokai) No. 2000-44389).
According to the methods proposed, a silicon single crystal having oxide precipitate nuclei which hardly shrink or vanish can be obtained by increasing the thermal stability of oxide precipitate nuclei in the crystal by doping it with nitrogen while growing it by the CZ method. It is alleged that oxide precipitate nuclei remaining in wafers sliced from such single crystal after the epitaxial step form oxide precipitates from the early stages of the device step and thus effectively serve as sinks for gettering, so that the effects of IG can be expected.
Later investigations, however, have revealed that thermally stable oxide precipitate nuclei which will not vanish even upon high temperature heat treatment can indeed be obtained by high concentration nitrogen doping of wafers but these oxide precipitate nuclei readily induce epitaxial layer defects. In other words, high concentration nitrogen doping results in the formation of stable oxide precipitate nuclei near the wafer surface but these nuclei induce stacking faults, dislocations and the like, namely epitaxial layer defects, in the epitaxial layer, which is the device active region. These defects cause an increase in the leakage current and degradation in the gate oxide integrity, among others.
In view of the epitaxial layer defect problem caused by conventional nitrogen doping, it is a primary object of the present invention to provide an epitaxial wafer scarcely showing epitaxial layer defect development as a result of suppressed growth of thermally stable oxide precipitate nuclei in spite of its being derived from a wafer sliced from a silicon single crystal pulled up in a nitrogen-doped form as well as a method of pulling up a silicon single crystal to serve as a raw material for such wafer.
To specify the temperature range in which thermally stable oxide precipitate nuclei are formed in nitrogen-doped single crystals, the present inventors made experiments in which a raw material silicon melt was doped with 1xc3x971014 atoms/cm3 of nitrogen and the pulling rate was varied during the step of pulling up to give silicon single crystals A and B having a diameter of 6xe2x80x3.
The concrete method of experimentation was as follows. A cylindrical portion was formed at an initial pulling rate of 0.7 mm/min and, at the time of arrival at a length of 500 mm, the pulling rate was reduced to 0.2 mm/min in the case of crystal A, or increased to 1.2 mm/min in the case of crystal B. Then, after growth to a cylindrical portion length of 550 mm, the pulling rate was again returned to 0.7 mm/min and the crystal was grown to 850 mm and then the pulling up is finished by tailing.
The thus-grown single crystal has a different thermal history depending on the change in pulling rate. For example, in the case of crystal A, the reduction in pulling rate results in slow cooling in the temperature range from the temperature at the start of rate reduction to a lower temperature side by about 100xc2x0 C. while, in the case of crystal B, the increase in pulling rate results in rapid cooling in the temperature range from the temperature at the start of rate increase to a lower temperature side by about 100xc2x0 C.
After pulling up in the above pulling rate changing experiments, samples were sliced from that portion of each single crystal which had been cooled within the temperature range of 1400xc2x0 C. (nitrogen concentration of 2.3xc3x971014 atoms/cm3) to 800xc2x0 C. (nitrogen concentration of 1.6xc3x971014 atoms/cm3) and measured for defect density. First, each sample was processed for lengthwise cleavage and the density of grown-in defects, namely void defects, which had been generated was determined. For void defect detection on that occasion, Bio-Rad""s defect detector OPP (optical precipitate profiler) was used, and that density was evaluated. Then, the sample was treated at 1200xc2x0 C. for 4 hours (as high temperature heat treatment) and subjected to selective etching (Wright etching) to 2 xcexcm and measured for heat treatment-induced defect density (oxide precipitate density) under an optical microscope.
FIG. 2 is a graphic representation of the relationship between the OPP defect density (grown-in defect density) and the temperature at the start of pulling rate changing as found in the pulling rate changing experiments. The relationship shown in FIG. 2 indicates that the density of grown-in defects increased in the temperature range of 1150-1050xc2x0 C. in crystal A while that density rather decreased in the same temperature range in crystal B.
The phenomenon seen in FIG. 2 teaches that the formation of grown-in defects occurs in the above temperature range. Thus, in crystal A, grown-in defects increase in size as a result of slow cooling and an increased number of defects were detected whereas, in crystal B, rapid cooling did not result in significant growth of grown-in defects and a low defect density was measured.
FIG. 3 is a graphic representation of the relationship between the etch pit density and the temperature at the start of pulling rate changing as found in the pulling rate changing experiments. As the results shown in FIG. 3 indicate, the etch pit density decreased in the temperature range of 1150-1050xc2x0 C. and increased in the temperature range of 1050-950xc2x0 C. in crystal A. In crystal B, on the contrary, the etch pit density increased in the temperature range of 1150-1050xc2x0 C. and decreased in the temperature range of 1050-950xc2x0 C.
As regards the phenomena in the two temperature ranges as shown in FIG. 3, firstly, the temperature range of 1150-1050xc2x0 C. is the temperature range in which grown-in defects are formed, as mentioned hereinabove, and the relevant phenomenon in crystal A indicates that vacancies were sufficiently consumed in grown-in defects and the subsequent growth of oxide precipitate nuclei was suppressed. In crystal B, on the other hand, the relevant phenomenon shows that since it was rapidly cooled in the same temperature range of 1150-1050xc2x0 C., grown-in defects were not formed to a sufficient extent but a large number of vacancies remained, so that the subsequent growth of oxide precipitate nuclei was promoted. Secondly, the temperature range of 1050-950xc2x0 C. is the temperature range in which thermally stable oxide precipitate nuclei grow, and, in crystal A, oxide precipitate nuclei were sufficiently grown as a result of slow cooling while, in crystal B, rapid cooling resulted in failure in oxide precipitate nucleus formation.
As discussed above, when a silicon single crystal is doped with nitrogen, those thermally stable oxide precipitate nuclei which will not vanish even upon high temperature heat treatment in the epitaxial step serve as epitaxial layer defect-causing factors. On the other hand, from the results shown in FIGS. 2 and 3, it becomes apparent that the temperature range in which thermally stable oxide precipitate nuclei grow as a result of nitrogen doping is 1050-950xc2x0 C. and that the cooling process in the temperature range of 1150-1050xc2x0 C. in which grown-in defects are formed influences the growth of oxide precipitate nuclei.
In other words, by controlling the thermal history in those temperature ranges in the step of single crystal pulling up, it becomes possible to suppress the growth of thermally stable oxide precipitate nuclei otherwise formed in the single crystal and thereby decrease the number of epitaxial layer defects.
The present invention, which has been completed based on the findings derived by analyzing the results of the above pulling rate changing experiments by the CZ method, consists of the following single crystal pulling up methods specified under (1) to (3) and the epitaxial wafer manufacturing method specified below under (4).
(1) A method of pulling up a single crystal from a silicon material melt doped with nitrogen while allowing the single crystal to grow, which method is characterized in that the period over which the single crystal passes through or remain in the temperature range of 1150-1050xc2x0 C. in the step of single crystal pulling up is not less than 50 minutes.
(2) A method of pulling up a single crystal from a silicon material melt doped with nitrogen while allowing the single crystal to grow, which method is characterized in that the period over which the single crystal passes through or remain in the temperature range of 1050-950xc2x0 C. in the step of single crystal pulling up is not more than 40 minutes.
(3) A method of pulling up a single crystal from a silicon material melt doped with nitrogen while allowing the single crystal to grow, which method is characterized in that the period over which the single crystal passes through or remain in the temperature range of 1150-1050xc2x0 C. in the step of single crystal pulling up is not less than 50 minutes and that the period over which the single crystal then passes through or remain in the temperature range of 1050-950xc2x0 C. is not more than 40 minutes.
(4) A method of manufacturing epitaxial wafers which comprises growing an epitaxial layer on the surface of a silicon wafer sliced from a single crystal pulled up by one of the methods mentioned above under (1) to (3).